Abstract

AIMS:

Radiation-induced disruption of mitochondrial function can elevate oxidative stress and contribute to the metabolic perturbations believed to compromise the functionality of the central nervous system. To clarify the role of mitochondrial oxidative stress in mediating the adverse effects of radiation in the brain, we analyzed transgenic (mitochondrial catalase [MCAT]) mice that overexpress human catalase localized to the mitochondria.

RESULTS:

Compared with wild-type (WT) controls, overexpression of the MCAT transgene significantly decreased cognitive dysfunction after proton irradiation. Significant improvements in behavioral performance found on novel object recognition and object recognition in place tasks were associated with a preservation of neuronal morphology. While the architecture of hippocampal CA1 neurons was significantly compromised in irradiated WT mice, the same neurons in MCAT mice did not exhibit extensive and significant radiation-induced reductions in dendritic complexity. Irradiated neurons from MCAT mice maintained dendritic branching and length compared with WT mice. Protected neuronal morphology in irradiated MCAT mice was also associated with a stabilization of radiation-induced variations in long-term potentiation. Stabilized synaptic activity in MCAT mice coincided with an altered composition of the synaptic AMPA receptor subunits GluR1/2.

INNOVATION:

Our findings provide the first evidence that neurocognitive sequelae associated with radiation exposure can be reduced by overexpression of MCAT, operating through a mechanism involving the preservation of neuronal morphology.

CONCLUSION:

Our article documents the neuroprotective properties of reducing mitochondrial reactive oxygen species through the targeted overexpression of catalase and how this ameliorates the adverse effects of proton irradiation in the brain.

Spine morphology and density in the DG of WT and MCAT mice. Spine morphology quantified at 6 weeks after proton exposure in WT (A) and MCAT (B) mice showed little effect of irradiation on either immature (long, mushroom) or mature (stubby) spine types. Overall spine densities did not change significantly after proton irradiation and were not found to differ between WT (C) and MCAT mice (D). All data are shown as the mean±SEM derived from all animals (n=3 animals, three sections per animal). DG, dentate gyrus.

Spine morphology and density in the CA1 of WT and MCAT mice. Spine morphology quantified at 6 weeks after proton exposure in WT (A) and MCAT (B) mice showed little effect of irradiation on long and stubby spine types. Trends (albeit not significant) were found for increased numbers of mushroom spine types in both genotypes after the 0.5 Gy dose. While differences between the genotypes were not found, overall spine densities trended higher in WT (C) and lower MCAT (D) mice after irradiation. All data are shown as the mean±SEM derived from all animals (n=3 animals, three sections per animal).

LTP of the dendritic fEPSPs in hippocampal slices from WT and MCAT mice after proton irradiation. LTP was induced by HFS at time “0” and recorded in CA1 neurons for 60 min. fEPSPs were evaluated by peak-to-peak amplitude by Mobius™ built-in algorithm. (A) LTP measured in WT mice at 3 months after proton exposure shows a marked radiation-induced variability in LTP expression associated with fluctuations in the fEPSPs. (B) LTP time course measured in hippocampal slices of irradiated MCAT mice shows that LTP was relatively unaffected by irradiation, as peak-to-peak variations in the magnitude of the fEPSPs showed considerable stability in the MCAT background and resembled unirradiated (0 Gy) controls. All data are shown as the mean±SEM derived from all animals (one to two slices per animal, n=16/cohort). fEPSPs, field excitatory postsynaptic potentials; LTP, long-term potentiation. HFS, high-frequency stimulation.

Western blot analysis of hippocampal synaptic AMPAR phosphorylation at 6 weeks after proton irradiation. (A) 0.5 Gy proton exposure increased the ratio of pGLuR1 (S831) to total GLuR1 in WT mice (*p<0.05), an effect that was abrogated in MCAT mice (*p<0.05). (B) There was no treatment effect found for either dose of protons with regard to pGLuR2 (S845) to total GLuR2; however, a genotype effect was observed at 0.5 Gy (*p<0.05). Data are presented as the average ratio of raw band intensity±SEM. n=6–8 per group. Black bars represent WT and white bars MCAT mice. AMPAR, AMPA receptor subunit.